In the field of power semiconductor technology, it is desirable to provide semiconductor devices with protection mechanisms that prevent destruction of the semiconductor devices under extreme switching conditions. Such extreme switching conditions arise because power semiconductor diodes are operated in commutation mode. When operated in commutation mode, high electric fields can occur, for example at the n−n junction of a pn−n semiconductor diode, which can lead to an avalanche-like generation of charge carriers at the n−n junction. At the same point in time, high electric field strengths can occur at the pn-junction of the pn−n semiconductor diode and lead to an avalanche-like generation of charge carriers at the pn− junction. The avalanche-like generation of charge carriers (so-called “avalanche generation”) results in an inability to maintain the high electric field blocking capability of the semiconductor diode in the n−-doped central region of the semiconductor diode. The semiconductor device thus loses its blocking capability and is destroyed unless external measures for limiting current and power have been implemented.
In order to avoid destruction of a semiconductor device such as a diode, the commutation process can be slowed down. When using such semiconductor diodes within insulated gate bipolar transistor (IGBT) semiconductor modules, however, such a slowdown can result in an increase in the switch-on losses of the IGBT. Other measures might lead to increased on-state or switching losses.
In US2007170514, a structure allowing an additional hole current is provided. To this end, additional p-doped regions are implemented in a field stop layer. By these p-doped regions directed towards a backside of the chip, an avalanche is created resulting in the desired hole injection. This provides for dynamically generated positive charges, or holes, to compensate for the incoming electrons.
Another variant is described in DE 102006046845 B4, where fixed positive charges are provided. During a normal shutdown operation, the reaction time of such p-regions is sufficient to cause an increase in softness and a reduction of the resulting switch-off during overload.
It has been shown that the previously described measures are generally not sufficient in cases of very rapid events such as, in particular, cosmic radiation events. The incoming radiation arrives in a narrow channel and thus causes an electron cloud, also called streamer. This phenomenon is associated with a field tip which rushes at high speed towards the cathode. According to simulations, such as described in W. Kaindl: “Modellierung höhenstrahlungsinduzierter Ausfälle in Halbleiterleistungsbauelementen”, Dissertation, Technical University of Munich, 2005, the last 50 μm, for example, are crossed in only 1 picosecond. The problem is that since the occurrence of an avalanche takes some time, the response time of the above described implemented p-zones is not sufficient for such rapid events. Charge carriers are accelerated in an electric field and create new free carriers by impact ionization of charge carriers. The velocity of charge carriers at high fields in Si is about 107 cm/s. The space charge zone for a structure in accordance with US2007170514 is, at a given blocking voltage of the pn-junction from about 10 to 100 V, in the case of the avalanche at only 10 V (as the lower limit provided for the design) at approximately a width w of 0.1 μm. The time required for a charge carrier to traverse this space charge zone is given by t=w/vs, which calculates to 1 picosecond, whereby the acceleration phase is neglected. Such a time delay is also described for avalanche-use IMPATT diodes, whereby the time delay is half the period of an oscillation.
A further adverse effect is the positive temperature coefficient of the avalanche effect, leading to an additional delay in the injection of holes, as the process triggered by the cosmic radiation event causes also a temperature increase.
For the above and other reasons, there is a need for a power semiconductor device with improved properties.